1. Field of the Invention
This invention relates to an equipment with shake sensing function such as a shake sensor, an apparatus with shake sensor and an equipment with shake correction apparatus such as a camera or a binocular, and so on.
2. Description of the Related Art
This type of equipment is described below by taking a camera as an example of the equipment with shake sensing function.
In a known shake correction apparatus used in the camera, a shake of a lens barrel or a camera body due to unintentional hand movement (camera shake) is detected by, for example, two angular velocity sensors respectively sensing angular velocities in two different directions. Undesirable high frequency noise component, drift, offset and so on are removed from angular velocity signals of the angular velocity sensors by a filtering operation. The angular velocity signals are integrated so as to be converted to signals corresponding to angles (hereinafter abbreviated as angular signals).
In order to reduce or cancel movement of an image on a film plane or on an image pickup device due to the camera shake, a correction lens is moved in a manner so that the optical axis of the correction lens is decentered in a direction perpendicular to the optical axis of the taking lens. Hereinafter, the movement of the image on the film plane is called "quantity of the shake". For calculating moving quantities of the correction lens or for deciding destination of the correction lens to be moved, the levels of the angular velocity signals are adjusted so as to be converted to corrected position control signals. The position of the correction lens is detected by position sensor(s).
When the driving device is driven, mechanical vibrations are transmitted to the correction lens. In the case of using an ultrasonic motor as the driving device, a stator in a rotary motor or a driving shaft in a linear motor is excitingly vibrated by driving signals having a frequency fp (hereinafter, called driving frequency fp). Thus, mechanical vibration having the same frequency as the driving frequency fp occurs in the correction lens, and so on.
In the angular velocity sensor, an excitation pulse generator generates excitation pulse signals having a predetermined frequency fg (hereinafter, called excitation frequency fg) which is generally in the ultrasonic region. A piezoelectric device is driven by the excitation pulse signals for excitingly vibrating an angular velocity detecting portion with the excitation frequency fg. The angular velocity sensor senses the angular velocity of the camera shake by using Coriolis force generated by an excitation velocity and a rotation component of the camera shake.
With respect to the filtering operation of the output signals of the angular velocity sensors, in order to cutoff noise signals having a fundamental frequency and harmonics of the excitation frequency fg of the angular velocity sensors and the driving frequency fp of the driving device, a cutoff frequency fc of a band-pass filter in the high frequency side is selected to satisfy the relations of fc&lt;fp and fc&lt;fg.
In the typical shake correction apparatus, the angular velocity sensors are provided in the vicinity of the correction lens and the driving device. When a mechanical vibration having the excitation frequency fg generated by the exciting vibration of the angular velocity sensing portion of the angular velocity sensor interferes with a mechanical vibration having the driving frequency fp generated by the driving of the driving device (cross modulation phenomenon), beat signals having frequencies fb newly occur further to the detection signals corresponding to the angular velocity component of the shake. Since the beat signals are superimposed on the actual detection signals of the shake, they cause a reduction in the output signals of the angular velocity sensors. The frequencies fb of the beat signals are generally defined by the following equation (1). EQU fb=.vertline.m.multidot.fg.+-.n.multidot.fp.vertline. (1)
Hereupon, the symbols m and n are respectively zeros or optional positive integers which never be zeros at the same time. When m.gtoreq.1 and n.gtoreq.1, the symbols m and n respectively designate the orders of the fundamental frequency or the harmonic of the vibrations having the frequencies fg and fp. The beat signals having the frequencies fb which are equal to or larger than the cutoff frequency fc of the band-pass filter in the high frequency side or the cutoff frequency fc of the low-pass filter can be removed by the filtering operation.
The conventional shake correction apparatus, however, is not considered to remove the beat signals by the filter. Thus, when the frequencies fb of the beat signals are smaller than the cutoff frequency fc of the filter, the beat signals cannot be removed by the filtering operation. The reduction of the S/N of the output signals of the angular velocity sensors due to the beat signals can never be improved. The reduction of the S/N of the output signals of the angular velocity sensors due to the beat signals occurs not only when the ultrasonic motors are used as the driving devices, but also when DC motors, voice coil motors or stepping motors are driven by pulse modulation drive such as PWM control (hereupon, the frequency of the pulse signals is assumed to be fp).